This invention is related to cameras and in particular to microwave cameras.
It is known that radar beams broadcasted from microwave radar antennas of a variety of designs can be effectively steered by imposing slight variations in the frequency of the beam being broadcasted.
The emitted microwave radiation from most objects over small frequency intervals in terms of frequency interval, temperature and emissivity can generally be approximated by: EQU P.sub..DELTA.f .apprxeq.K.epsilon.T .DELTA.fwatts.multidot.cm.sup.-2 .multidot.ster.sup.-1
Where
K is a constant, PA1 .epsilon. is the emissivity relative to blackbody radiation PA1 T is absolute temperature, .degree.K., PA1 .DELTA.f is the frequency interval in Hz, and PA1 P.sub..DELTA.f is the power of the radiation. PA1 T.sub.1 =thermometric temperature of body, .degree.K., PA1 T.sub.2 =radiometric temperature of an object whose radiation is reflected by the body, .degree.K., PA1 T.sub.3 =radiometric temperature of any object whose radiation is transmitted through the body .degree.K., PA1 r=reflectivity, and PA1 .tau.=transmissibility
Thus, for a given .DELTA.f the emitted power is proportional to .epsilon.T for the body and is typically not a function of frequency. In addition to emitted radiation most bodies to some extent reflect microwave radiation from the surroundings and some bodies which are transparent will transmit energy from objects behind it.
A concept of radiometric temperature is useful in considering microwave systems. The radiometric temperature of a body is defined as being equal to the thermometric temperatures of an ideal black body that would give the same radiation as that emanating from the body. A convenient expression for the radiometric temperature is: EQU T=.epsilon.T.sub.1 +rT.sub.2 +.tau.T.sub.3
in which
At equilibrium, .epsilon.=(1-r-t)so EQU T=(1-r-t)T.sub.1 +rT.sub.2 +.tau.T.sub.3
For water .epsilon. is close to 1, for metallic objects r is close to 1, and for many natural objects t is close to one. For most objects .epsilon., .tau. and r change slowly with the frequency of microwave radiation over most of the spectrum. For many outdoor situations T.sub.1 will be in the range of about 300.degree. K. and T.sub.2 representing the temperature of of the sky will be in the range of a few degrees K. (say 10.degree. K.). Thus, for an outdoor target comprised of aluminum foil reflecting the sky with r of about 1.0 mounted on opaque non reflecting background, T.sub.B /T.sub.A would be: ##EQU1## where EQU T.sub.B =temperature of the background and, EQU T.sub.A =temperature of the aluminum foil.
Thus, microwave radiation emanating from the non-reflecting background should be about a factor of up to about 30 greater than that emanating from aluminum foil reflecting the sky.
Spectrum analyzer systems for microwaves are well known and are commercially available. It is known that acoustic waves can be generated in a Bragg cell to produce diffraction patterns capable of deflecting a laser beam.